The present invention is generally related to lithographic duplicators and, more particularly, to a versatile system for controlling ink and moisture feed rates during printing.
The production of quality copies by lithorgraphic means requires that the ink and moisture each be supplied at a rate proper for the demands of the lithographic master. It is also necessary that a proper balance be maintained between the ink and moisture at all times. If the amount of ink or moisture, or the balance therebetween, is not maintained within predetermined ranges, noticeable copy degradation will result. For example, excessive moisture or an excessive moisture/ink ratio will reduce the ink transferred, resulting in copies with low optical density image areas. On the other hand, low moisture or excessive ink will cause the image areas to be blurred and may result in background toning.
In general terms, one of the primary problems over the years has been that the ink and moisture requirements for producing quality copies vary significantly with changes in operating and environmental conditions. For example, variations in temperature and humidity will change the amount of moisture required by the master for quality copies. Also, certain plates or masters, both referred to herein generally as "masters" such as those of the zinc oxide type, undergo changes during copy runs which have an effect upon the amount of moisture required. The moisture/ink requirements also may be affected by the presence of additional moisture introduced into the system as new masters are loaded in sequence for relatively short copy runs, wherein each new master is "wet" and adds moisture to the system.
The particular problem addressed by the present invention is that of moisture lost during machine shutdowns. This is attributable primarily to evaporation which occurs during the shut-down period. There is a direct relationshp between the amount of moisture lost and the duration of the shutdown. With conventional lithographic duplicators, it was necessary for the operator to manually increase the moisture feed rate momentarily upon restart following shutdown for one reason or another. Depending upon the circumstances, it was sometimes also necessary to reduce the ink feed rate slightly since the dryer ink was more transferable. These adjustments were further complicated by the fact that the amount of evaporation was significantly less when operating a "cold" machine, as opposed to a "warm" machine where higher evaporation rates were anticipated. For the most part such manual adjustments were "hit or miss" and often did not produce satisfactory results, particularly for unskilled operators.
In order to free the machine operator from the need to make such "hit or miss" adjustments, it would be desirable to have a control system which adjusts the ink and moisture feed rates to compensate for losses in moisture during shutdowns.
Therefore, it an object of the present invention to provide a novel control system for lithographic duplicators which adjusts the moisture feed rate to compensate for moisture losses during machine shutdowns, without the need for the assistance of a skilled operator.
Another object of the present invention is to provide a unique control system for a lithographic duplicator which adjusts the ink feed rate to compensate for moisture losses during machine shutdowns which result in drier, more transferrable ink, and to make such adjustments without the assistance of a skilled operator.
It is a further object of the present invention to provide a versatile control system which adjusts both the ink and moisture feed rates to compensate for losses in moisture during machine shutdowns to provide the proper ink/moisture balance upon restart, whereby satisfactory copy quality is maintained without special operator assistance.